Proposal: allow PresenceRouter full control over presence targets / more presence settings

This issue has been migrated from [#10423](https://github.com/matrix-org/synapse/issues/10423).

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**Description:**
Currently, PresenceRouter can only _add_ interested parties on top of the default "everyone I share a room with" policy. I think it would make sense if it gets to completely decide the interested parties. It then becomes possible to exclude some rooms, don't do presence over federation (but do it locally), only if you share a small/private room, et cetera. It would basically allow for some analogs of limit_remote_rooms but then for presence. Some of these could also be made easily accessible in the settings.

**Motivation:** I am in #homeowners:matrix.org which is a "broadcast room" with a _lot_ of servers. I don't like my presence to be sent to the people who I only share that room with because it throws my home network in a small crisis every time my presence gets sent, while I am sure that none of the recipients is interested in my presence. (And I also like my privacy.)

Is this something you would consider? I'm happy to write a PR.

EDIT: to be clear
Current behavior: PresenceRouter ∪ "everyone I share a room with"
Proposed behavior: PresenceRouter. (with the default router being "everyone I share a room with")